Fluorescent

A fluorescent pigment/dye is one that absorbs light at one wavelength and emits it at another. This is the phenomenon used in blacklight illumination, which is simply a UV Illuminant that is invisible to the human eye when simply reflected off of a surface (e.g. skin in the below image, which is not fluorescent), but is shifted to a higher wavelength (i.e. in the visible range to get the desired effect) when fluoresced.

See also Phosphorescence, which is the same phenomenon on larger time scales.

Fluorescent lights are Illuminants that use fluorescence to emit light. Instead of being excited by a photon, they are excited by electricity. Regardless of excitation though, that pigment will fluoresce and emit light. Fluorescent are disadvantaged by having low CRI, which means colors illuminated by fluorescent lights can be distorted in way that simple White Balance cannot correct.

When describing a color, fluorescent is not a quality of the color itself, but rather describes that the fluorescent color is much brighter than would normally be expected, relative to its surroundings. Where a typical yellow may be able to reflect ~90% of the illuminant’s yellow light, a fluorescent yellow may be reflecting/re-emitting as much as 300% of the illuminant’s yellow light, as long as the Illuminant’s Spectrum also has a high UV content relative to its visible (yellow) content, such as with natural daylight.